This invention relates to phosphor particles having ultrathin coatings on their surfaces and to methods for making and using such coated particles.
Phosphors are used in flat panel plasma displays (FPDs), cathode ray tubes, x-ray imaging devices, field emission devices, fluorescent lighting fixtures, and a variety of other applications to generate visual images or simply provide light. Although a wide variety of phosphor materials are known for use in these applications, those materials all have in common the ability to generate a characteristic light in response to exposure to an excitation energy source. The excitation energy source may be, for example, a photon (photoluminescence, or PL), high energy electron beam (cathodic luminescence, or CL), an applied electrical field (electroluminescence, or EL), or applied heat (thermoluminescence). Electroluminescent phosphors are of particular interest for flat panel display applications.
Flat panel displays commonly include a phosphor layer which is sandwiched between two insulator layers. A reflecting electrode such as aluminum and a transparent electrode such as indium tin oxide bracket the phosphor/insulator sandwich. In the standard MISIM (metal-insulator-semiconductor (phosphor)-insulator-metal) construction, a glass or other transparent substrate lies atop the transparent electrode. Various filter layers may be incorporated into the structure to assist with color production.
The phosphor material is commonly printed as a thin film onto an adjacent layer. There are several reasons that make it preferable to use a powdered phosphor. Chief among these is cost—powdered phosphors can be used in very small amounts and so the amount of phosphor that is needed can be significantly reduced. In addition, light loss through internal reflection can be minimized using particles and there is no loss in brightness due to light lost at edges, as in thin phosphor films. Efficiency (light emitted/unit applied power) is also higher for powders. The use of powders also makes it possible to produce all colors in a single phosphor plane, as a particulate mixture of different color-emitting phosphors can be formed as a single layer.
The phosphor particles typically are composites of a host material that in the case of electroluminescent particles provides a necessary set of electrical properties, and one or more “luminescent centers”. The “luminescent centers” are usually metal cations and sometimes anions which are “doped” or otherwise combined with the host material. These ions usually become incorporated into the crystalline lattice of the host material, or dispersed as discrete domains within the host material. The luminescent centers provide the desired optical emission properties to the phosphor particles. Again, a wide variety of these materials are known, which differ in their composition according to the specific application and desired emitted color. Phosphors that emit white, yellow, red, green and blue wavelengths of visible light are commonly used in display and monitor applications.
It is often necessary to coat the surface of the phosphor particles. Reasons for doing this include (1) particle protection, often against reaction with water, but also against reaction with air, other oxidants, or contaminants; (2) improving screening characteristics and (3) improving contrast or pigmentation. Among the coating materials used for these purposes are ZnO, MgO, In2O3, Al2O3 and SiO2, and CuS. Chemical vapor deposition (CVD) and sol-gel methods have been used to provide coatings of these types.
To be effective, the applied coating needs to be as uniform and as thin as possible. It is also beneficial that the coating process does not cause individual particles to agglomerate to form larger aggregates. In addition to having much larger diameters than are wanted, these aggregates often tend to break apart, revealing defects in the coating at the break areas. The underlying particles are subject to attack from water, oxidants and other materials at the places where these defects occur. Neither CVD nor sol-gel techniques are entirely satisfactory, as agglomerates tend to form readily in these processes. In addition, these methods require relatively large amounts of raw materials, as only a portion of the applied reactants actually become applied to the surface of the phosphor particles. Quite often, material applied by these processes form separate particles instead of forming films on the surface of the phosphor particles. For various reasons, it is desired to develop phosphor particles that are smaller than those commonly used now. Commercially available phosphor particles usually have diameters in the 1-50 micron range. Phosphor particles having diameters of less than 1 micron, and in particular less than 100 nm, potentially offer advantages in screen design and performance. CVD and sol-gel coating methods are particularly unsuitable for coating these smaller particles.
It is therefore desirable to provide a method by which thin, conformal coatings can be applied to the surface of phosphor particles. Such a method preferably will result in minimal agglomeration of the particles, and provide coatings with minimal defects. The method also desirably permits very thin coatings to be applied, and further allows for close control over coating thickness. Even more preferably, the method would allow for sequential deposition of multiple coatings of different materials onto the phosphor. For example, a preferred method would permit one to deposit a luminescent layer atop a particle of the host material, and then to deposit a protective layer atop the luminescent layer.